The present invention relates to a control technique of servo signal detection in which even if relative positional relation of a plurality of heads, which are built into a hard disk drive, with a servo signal area of a disk corresponding to each of the heads deviates, performance of the hard disk drive does not decrease.
In recent years, for the hard disk drive (HDD), a magneto-resistive (MR) head and a giant magneto-resistive (GMR) head are adopted as a magnetic head, which results in sharply increased recording density of the drive. Concerning the magneto-resistive head and the giant magneto-resistive head, in order to increase detection sensitivity, a R/W IC supplies an electric current having high density to a thin film sensor having magnetoresistance for use. For this reason, as preventive measures against electro-migration of a head sensor, data placement (data format) is performed so that usually an electric current is not supplied to one head for a long time continuously, and so that operation for switching a head (head change) is performed comparatively frequently.
In addition, as recording density of data becomes high, recording density of a servo signal area in a disk circumference direction becomes higher. An area length of a servo signal area in a disk circumference direction tends to be narrowed. A length of a servo signal area is currently several tens of μm per area. Additionally, in order to improve positioning performance, it is important to widen a servo detection frequency band width. Therefore, it is necessary to increase the number of servo signal areas per circuit.
Under the circumstances, as a servo signal area is narrowed, and as the number of servo signal areas per circuit increases, the following problem arises: when head change occurs, detection timing deviates from an expected servo signal area. To be more specific, as shown in FIGS. 13(A) and 13(B), a spindle leans due to change in thermal environment of HDD and an outside shock (for example, a center of an actuator shaft inclines, causing a position gap between a head #0 and a head #7 of an actuator); and distortion caused by a clamp (a spindle fixture, not illustrated) of the disk is released by a heat history or an outside shock, resulting in a slip in a circumference direction of the disk.
Because of it, if a servo signal area deviates from an expected area at the time of head change, a timing gap (skew) occurs. To be more specific, at the time of head change, if a servo signal area of a track corresponding to a head after change deviates from that before the change as shown in FIGS. 13(A) and 13(B), proper detection timing of the servo signal area after the change deviates. As a result, normal servo detection cannot be expected. Moreover, there is a high possibility that performance is extremely decreased due to frequent retry processing.
In order to avoid such problems caused by skew of a servo signal area 13, as shown in FIGS. 13(A) and 13(B), the following measures are taken: a technique for adding an area corresponding to the relatively possible amount of head skew to a position before the servo signal area 13, as an increment 18 of the servo signal area; eliminating distortion of a clamp by adding a sufficient heat history before writing a servo signal on the disk by a STW (servo track writer); and the like.
The former example of measures for avoiding the problems of skew (the example in which the increment of the servo signal area is added) will be described more specifically. As shown in FIGS. 13(A) and 13(B), if it is based on the assumption that a lean range 17 is ±15 μm for the head #7 (H7) (30 μm for a width), an acquisition signal of 30 μm for a servo signal is added as an increment 18 before all servo signal areas 13 of all heads. Then, as is the case with SGATE (control signal for detecting a servo signal) 19-1, 19-2, it is always detected while moving forward by 15 μm. As shown in FIGS. 13(A) and 13(B), if H7 deviates by 15 μm, when performing head change from H7 to H0, SGATE 19-1 opens at the top of an increment area of H0; and when performing head change from H0 to H7, SGATE 19-2 opens at the end of an increment area of H7. Therefore, servo detection for both becomes possible without hindrance.
If it is described at full length with reference to FIG. 13(B), as measures against a shift of a head (or a disk) in a track direction on the upper and the lower sides, the increment 18 of the servo signal area is additionally written in all tracks so that it is adjacent to the original servo signal area 13. In this case, in order to read a servo signal in the servo signal area 13 or in the increment 18, it is so devised that a servo gate signal (indicated as 19-1 or 19-2 in FIG. 13(B)) is opened to distinguish the servo signal from data before reading the servo signal. In FIG. 13(B), for example, an increment having a length of 30 μm is a length for which the forward and backward shifts with reference to the track direction (circumference direction of the disk) are taken into consideration. In actuality, it is either a forward shift or a backward shift with reference to the track direction. Therefore, it is so devised that a servo gate (indicated as SGATE in the figure) is opened in a substantially central part of the increment 18 to cope with shifts in both directions.
In addition to it, in the above description, the head change from H7 to H0 was assumed to be a change to a servo signal area written on the same radius of the disk. However, the head change is not limited to this. As a matter of course, the head change may be a change to the next servo signal area that is separated by one data block (actually, in many cases, only an interval T between servo signal areas is offset to perform a head change).
As described above, providing the servo signal area increment 18, and adjusting timing of a servo gate, prevent a timing gap from occurring at the time of a head change.